There were early placentals in Australia. I believe a condylarth tooth was
found in Eocene sediments. The migration path was probably from South
America via Antarctica. Those died out, and the next placentals were bats
in (I believe) the Miocene. Later, rodents appeared, but the contacts were
apparently via a 'sweepstakes' route from Sundaland (Indonesia). About
60,000 BP _modern_ H. sapiens appears, complete with art, apparently via
New Guinea, where there is evidence for some sort of sea-going culture at
that early date. Dingos appear later.

A couple of interesting points: the earliest evidence for modern human
behavior (art, sea exploration) are these data from Australia and New
Guinea. Second, these early cultures were very definitely transitional in
multiple senses, both to archaic H. sapiens and (possibly) to very late H.
erectus. Even today, some North Australian native groups have (female)
endocast volumes in the low H. erectus range (about 750 cc). Those folks
are fully modern in culture, behavior, and intelligence. Think about that!